LONDON, Ohio — Aided by a computer and a Global Positioning System — a constellation of Earth-orbiting satellites — a farmer's tractor can now drive itself.

The use of such technology has increased in the past few years as farmers try to cut costs to stay profitable as crop prices remain relatively stagnant. Infrared sensors control how much fertilizer is applied. Retinal imaging tracks cattle. On the horizon, perhaps, are tomato-picking robots.

Experts estimate that up to 15 percent of farmers now have GPS precision-controlled tractors or combines.

"It's the difference between making money and not making money," said Dave Mowitz, machinery and technology editor for Successful Farming, a national farm publication based in Des Moines, Iowa.

AutoFarm, a company based in Menlo Park, Calif., adapted a $40,000 GPS system for agriculture from an automatic aircraft-landing system. About 300 have been sold since the equipment went on the market last spring.

"Everybody's been looking at that and saying, 'Gosh, can they do it?'" said Thomas Wagner, who was helping demonstrate the GPS precision-curve system at the Farm Science Review near this central Ohio city in September. "Bring your tape measure and we'll show you we can actually do the curves."

GPS receivers placed on top of tractors pull in locational radio signals from satellites and a ground station fed by satellites. A computer inside the tractor memorizes the coordinates of the field and guides the tractor over the same path — for tilling, planting, spraying and harvesting.

Expense is one reason high-tech can be a hard sell to farmers. Some believe that precision-guidance systems make economic sense only for large farms.

"They are slow to adapt," said Joe Malone, who raises hogs near Lancaster and formerly worked in robotics for Goodyear Corp. "Once you get a farmer that looks at what it can do for him from a standpoint of money — that's all he's looking at. Then he'll buy into that project right now."

GPS systems, which began hitting the fields about seven years ago, can cut the loss of herbicide to evaporation by allowing farmers to work at night, when winds often are calmer. And driving over the same route each year minimizes the compaction of soil by the wheels that can reduce yields.

"It allows them to farm 24 hours a day if they want to," Mowitz said. "It can be pitch black, and you can be planting your fields."

Dennis Hancock, an agriculture researcher at the University of Kentucky, said precision-guidance systems can save farmers as much as 5 percent in fertilizers and pesticides.

More high-tech tools
Another technology, remote sensing, can reduce the use of fertilizer and minimize runoff pollution. Sensors got their start in the 1980s, but their use has taken off in the past five years.

Photodiode sensors mounted on a sprayer absorb the color spectrum being reflected by the plants, and a computer determines how much fertilizer or herbicide to spray.

"You're putting more on plants that can use more and not putting as much on plants that can't use as much," Hancock said. "There is sometimes a pretty tremendous economic benefit in that."

Computer/GPS systems also produce maps showing where a field is the most and least productive. Farmers can use more seed and fertilizer in less productive areas or take those areas out of production.

As a combine moves through the field threshing corn and spitting it into a bin, a sensor in the bin measures the output while the GPS system marks and records the crop yield in each spot.

Farmers in Kentucky — where the rolling terrain can make for both high and low production in the same field — have saved as much as $30 an acre by using the yield monitors, Hancock said.

The cattle-identification system uses a high-speed digital camera to illuminate and scan the retinas of calves or lambs. The images are loaded into a computer, and a GPS signal tracks the animals from the pasture to the slaughterhouse.

The equipment, which costs about $2,700, has been in use for about a year.

John Cravens, marketing director for developer Optibrand Ltd. of Fort Collins, Colo., said the system could help quickly identify an animal with mad cow or other diseases. It also provides a profile of each animal, including weight, diet and medical treatment.

For some, it's all a bit much.

Not for small farms
John Ikerd, professor emeritus of agricultural economics at the University of Missouri, said the new technologies will result in larger farms and fewer farm families.

Decisions about what and how much to plant will be made by faraway corporate officers whose primary focus is maximizing return to shareholders and not necessarily what is good for the land, he said.

More technologies are on the way. Not yet in the fields is the tomato-picking robot.

Developed by Ohio State University researchers with a $100,000 grant from NASA, the single-armed robot has a prosthetic hand and a lipstick-sized camera and is mounted on a portable platform.

Researcher Peter Ling said the camera spots tomatoes and determines their size and color even if they are partially hidden behind leaves or stems. Using a suction cup and fingers that employ a loose grip, the hand plucks the tomatoes from the vine and drops them into a bin.

Malone, the Lancaster farmer, said corporate farms might start using such robots in the next two or three years to sort good tomatoes from bad and save money on labor.